They aren't Derek Jeter, Jorge Posada and Alex Rodriguez, penalized only for their consistency. They aren't Roger Clemens, for sure, and Bobby Abreu, for all his alarmingly eerie win/loss splits, is there either.
It's these three:
Melky Cabrera
Through May 29 (Yanks' record: 21-29): 40 games, 147 PA, .215/.278/.300
Since May 29 (Yanks' record: 41-21): 61 games, 258 PA, .336/.383/.498
Mariano Rivera
Through May 29: 1-3, 5.94 ERA 16 G, 3 Sv, 16.2 IP, 18 H, 4 BB, 18 K, .273/.314/.455
Since May 29: 1-0, 1.20 ERA 24 G, 14 Sv, 30 IP, 25 H, 1 BB, 29 K, .227/.252/.255
Luis Vizcaino
Through May 29: 2-1, 7.27 ERA 25 G, 26 IP, 24 H, 21 BB, 13 K, .247/.380/.443
Since May 29: 6-1, 1.16 ERA 32 G, 31 IP, 20 H, 13 BB, 31 K, .182/.264/.264
There are other contributors, for sure -- oddly enough, Jason Giambi's injury being either a huge coincidence or somehow a factor. But these three have been essential to the Yankees ripping off the absolutely necessary 20-7 run that the team is currently on.

Melky deserves to be the everyday center fielder. I hope Torre realizes that and keeps him playing.
I wholeheartedly agree. It's not favoritism toward anyone but the best player, and Cabrera in 2007 has been superior afield and at the plate.